Layer the base: In your slow cooker, add sliced onions and mushrooms, then sprinkle in the minced garlic. These create a flavorful bed for the sauce.
Mix the sauce: In a bowl, whisk together the cream of mushroom soup, beef broth, Worcestershire, Dijon, paprika, pepper, and salt until smooth.
Add meatballs: Place the frozen meatballs on top of the vegetables, then pour the sauce evenly over everything.
Slow cook: Cover and cook on Low for 5–6 hours or High for 2.5–3 hours, until the meatballs are heated through and the onions are tender.
Thicken if needed: If you prefer a thicker sauce, stir the cornstarch and water together, then mix it into the slow cooker.
Cook on High for 10–15 more minutes until slightly thickened.
Make it creamy: Turn the heat to Low (or switch the slow cooker to Warm). Stir in the sour cream until the sauce is silky and combined. Do not boil after adding sour cream.
Cook noodles: While the sauce finishes, cook egg noodles according to the package.
Drain well.
Serve: Spoon the creamy meatball stroganoff over the noodles. Finish with fresh parsley for color and brightness.